Hello Sir ... I M 23 Yr Old(unmarried)my Last Period

hello sir ...
i m 23 yr old(unmarried)my last period date was 5th December... i m regulary menstruating..28 day cycle nd i had intercourse on 26 27 28 with using condom ..nd condome didn't spillage...
nd then 3rd jan was my expectin period dat but I couldn't..so i hadd taken norethisteron 5mg tablet on 10:00 am 4th jan nd 2nd tablet on 10:00 am 5th jan ...but after that periods doesn't start...then 7th jan today test for pregnacy.."i can"pregnancy test ..that's is negative nd from 2 days mild lower abdominal cramp is...
i think i hv a stress ...or i m shifiting to the hostel so diet changes ..i m vegetarian but last month non veg was eaten during period of intercorse...what was the reason ??
what should i do ?? ...wait or take another course of norethisteron ..when nd dose ????